Side force for an air vehicle is conventionally produced when flying the air vehicle in sideslipping flight, at a sideslip angle of attack β, often referred to simply as sideslip angle β. Sideslip angle β relates to a yaw rotation of the air vehicle centerline from the relative wind direction, and is conventionally regarded as “positive” when the relative wind is coming from the starboard side (right of the nose) of the air vehicle, and “negative” when the relative wind is coming from the port side (left of the nose) of the air vehicle. Side force for an air vehicle is also conventionally produced when deflecting a rudder of the air vehicle. Also conventionally, rudder deflection (δr) is considered “positive” when the rudder is deflected towards the port side (left of the vertical tail) of the air vehicle, and “negative” when the rudder is deflected towards the starboard side (right of the vertical tail) of the air vehicle.
The main contributors to the side force at sideslip are the fuselage (or body) and the vertical tail of the air vehicle. These two components produce yawing moments of opposite signs, defining the level of directional stability of the air vehicle configuration. The resulting yawing moment when the air vehicle is conventionally trimmed at sideslip flight by rudder deflections, produces a significant loss of initial untrimmed side force.
For example, referring to FIG. 14(a), in such sideslipping flight, the air vehicle is illustrated at a positive sideslip angle β, and the fuselage and vertical tail together generate a net positive side force in the port direction. The vertical tail also induces an accompanying net clockwise yaw moment which if untrimmed would tend to stabilize the air vehicle and align the air vehicle centerline with the relative wind direction, thereby diminishing the sideslip angle β and net side force, eventually both to zero. Conventionally, the air vehicle may be trimmed to maintain the sideslip angle β by inducing a counter-clockwise yaw moment, and this is conventionally done by providing a positive rudder deflection as illustrated in FIG. 14(b). However, this also results in the original untrimmed net side force being significantly reduced.
Air vehicle configurations having a tandem arrangement of a forward vertical surface as well as an aft vertical surface (on longitudinally opposite sides of the center of gravity) can theoretically help trim the undesired yawing moments while producing increased values of side force in the desired direction, and may be capable of producing trimmed side force even at zero sideslip angle of attack, using simultaneous deflections of forward and aft rudders. However such an air vehicle configuration inherently has reduced directional stability as compared to a similar air vehicle configuration in which the forward vertical surface is missing, or, alternatively, requires a significantly larger aft vertical surface than forward vertical surface for maintaining directional stability.